read-tree: add performance test in p2000

Signed-off-by: Victoria Dye <[email protected]>

Author: vdye

t/perf/p2000-sparse-operations.sh

@@ -112,6 +112,7 @@ test_perf_on_all git commit -a -m A
 test_perf_on_all git checkout -f -
 test_perf_on_all git reset
 test_perf_on_all git reset --hard
+test_perf_on_all git read-tree -mu HEAD
 test_perf_on_all git checkout-index -f --all
 test_perf_on_all git update-index --add --remove
 test_perf_on_all git diff